Method of treating alloys



Patented Sept. 25, 1934 UNITED STA This invention relates'to aluminum bywhich is meant inrau'r OFFICE METHOD or 'rnca'rme armors Alailal' P801,Cleveland, Ohio,-

Company of America, a corporation of Pennsylvania Application z,- 1932,Seria1 m cerman o ne a, 1931,

No Drawing? assignor .to Alu- Pittsburgh, Pa.,

base alloys, alloys containing more than 50 per cent of aluminum, whichcontain large amounts of silicon,

and to treatments by which 5 alloys of this nature may be improvedforthe purposes for which theyare used.

Aluminum base alloys containing silicon in" large amounts, thatis, inamounts of 16 per cent or above, are useful because of their lowheatexpansivity. Particularly are they useful as a mafrom which pistons andother machined terial parts which are submitted to movement at hightemperature can be made.

Such alloys, however,

have not heretofore met all of the requirements which such commercialuses demand, and because of the large amount of silicon present in suchalloys, such amount being above the eutectic composition, machiningdifliculties are greatly increased. Aluminum-silicon alloys containing16 per cent or more of silicon have not heretofore possessed thecombination of high hardness at a.

high temperature and low heat expansivity with good machining propertiesand good strength.

The object of the present invention is to provide means by which analloy having the above combination of properties may be made. Anotherobject of the invention is to produce alloys of such characteristicswhich may be readily used for commercial purposes.

Other objects of the invention will appear in the following descriptionthereof.

I have discovered that if an'aluminum' base alloy containing about 16 to25 percent of silicon,- about 0.1

to 3 per cent of nickel,.about 01 to 1.25 per cent of magnesium, andabout 0.1 to 1.75 per cent of copper, preferablyabout 0.1 to

1 per cent of copper, is treated in the molten condition by the additionthereto of certain modifyi%;elements and is thereafter cast in permanentmolds, the casting thus markable combination of high hardness atproduced has a rehigh temperature, low thermal expansivity, goodmachining properties and good strength.

Because of the fact that these alloys have a silicon content above theeutectic composition, large amounts of primary silicon crystals separatein the alloys manner. ing treatment tion of these I have it is possibleto prevent the separawhen they are cast in the usual found that by aspecialmodifyprimary silicon crystals. I have likewise found that thetreatment which produces this effect also induces in the alloy the novelproperties above mentioned. In treating the alloys in accordancewith myinvention,

in the molten I place them condition and add to'the melt an amount of analkali metal such as sodium, potassium, rubidium, or caesium, or a saltor salts of alkali metals which will in contact with the molten aluminumproduce an alkali metal. The amount of alkali metal introduced into themelt by the (39 treatment may vary from 0.05 to as high as 1 per cent.When larger amounts than about 0.1 per cent by weight of an alkalimetal, such as sodium, are to be added, I prefer to first cover themolten alloy with a molten salt mixture since I have found that thedesired effect is more readily obtained by the use of a molten saltcovering on, the alloys above described.

The molten alloy, having been thus treated, is then cast into apermanent mold, i. e., into a mold, usually made of iron or steel, thewalls of which afford means of conducting the heat rapidly. from themolten metal poured therein.

The casting having been made in this manner, I then heat the casting, inthe preferred practice of my invention, between about 100 and 150centigrade, since I have found that by this heating, the hardness,strength and machining characteristics of this alloy are distinctlyimproved. While it is not necessary, in order to obtain the advantagesabove noted, to practice this heating step, I have found it verydesirable to so do, and when such heating is practiced I have found itnecessary to heat the alloys for at least 4 hours and as much as 24hours'in order to produce an alloy having the desired combination ofhigh hardness and strength, good machining characteristics and lowthermal expansivity. i I

As an example of the practice of my invention, an aluminum base alloycontaining 20 per cent silicon, 3 per cent nickel, 0.8 per centmagnesium, and 1 percent copper, together with the usual ironimpurities, was melted. The molten alloy was then covered with a fluxcomposed of 5 parts of calcium fluoride, 6.5 parts of sodium carbonate,and 3 parts of a mixture of 60 per cent sodium chloride and 40 per centpotassium chloride. The proportion of this mixture amounted to about 1.5per cent of the weight of the treated alloy. After this flux was melted,metallic sodium in amount of about 0.15 per cent by weight of the totalmelt was introduced into the molten alloy. The alloy was then cast intoan iron mold and the resultant casting was heated for 24 hours at150.centigrade. The thus treated casting had a hardness of 150 Brinellat. room temperature and 140 Brinell at centigrade. The alloy had astrength of 37,500 pounds per square inch and the coefficient of thermalexpansion of the alloy was below 19 x 10* per. degree centigrade. Thecast- 0 ing had excellent machining characteristics and wascharacterized by an absence -of primary silicon crystals.

Having thus described my invention, I claim:

1. A method of producing castings of high hardness and low thermalexpansivity by adding to a molten aluminum base alloy containing 16 to25 per cent of silicon, 0.1 to 3 per cent of nickel, 0.1 to 1.25 percent of magnesium, and 0.1 to 1.75 per cent of copper, a metal of thegroup consisting of sodium, potassium, rubidium and caesium, andthereafter casting the molten alloy in a permanent mold.

2. A method of producing castings of high hardness and low thermalexpansivity by adding to a molten aluminum base alloy containing 16 to25 per cent of silicon, 0.1 to 3 per cent of nickel, 0.1 to 1.25 percent of magnesium, and 0.1 to 1.75 per cent of copper, a metal of thegroup consisting of sodium, potassium, rubidium and caesium, casting themolten alloy in a permanent mold and thereafter heating the resultantcasting at a temperature of 100 to 150 centigrade for at least 4 hours.

3. A method of producing castings of high hardness and low thermalexpansivity by covering a molten aluminum base alloy containing 16 to 25per cent of silicon, 0.1 to 3 per cent of nickel, 0.1 to 1.25 per centof magnesium, and 0.1 to 1.75 per cent 01 copper with a molten saltflux, adding to the thus covered molten alloy a metal of the groupconsisting of sodium, potassium, rubidium and caesium, and thereaftercasting the molten -alloy in a permanent mold.

4. A method 01' producing castings of high hardness and low thermalexpansivity by covering a. molten aluminum base alloy containing 16 to25 per cent of silicon, 0.1 to 3 per cent of nickel, 0.1 to 1.25 percent of magnesium, and 0.1 to 1.75 per cent of copper with a molten saltflux, adding to the thus covered molten alloy a metal of the groupconsisting of sodium, potassium, rubidium and caesium, casting themolten alloy in a permanent mold and heating the casting thus producedat a temperature of about 100 to 150 centigrade for at least 4 hours.

ALADAR PACZ.

